Iproto call won't leak if transaction isn't committed

In case of throwing client error because of inactive function
we did not destroy used port. It could possibly cause huge
memory leaks as could be seen with top or its analogues when
performing net.box test run in a loop.

Closes #4388

src/box/call.c

@@ -132,6 +132,7 @@ box_process_call(struct call_request *request, struct port *port)
 
 	if (txn != NULL) {
 		diag_set(ClientError, ER_FUNCTION_TX_ACTIVE);
+		port_destroy(port);
 		txn_rollback(txn);
 		fiber_gc();
 		return -1;